PCI Hotplug: acpiphp: grab refcount on p2p subordinate bus

If a logical hot unplug (remove) is performed on a bridge claimed
by acpiphp and then acpiphp is unloaded, we will encounter an oops.

This is because acpiphp will access the bridge's subordinate bus,
which was released by the user's prior hot unplug.

The solution is to grab a reference on the subordinate PCI bus.
This will prevent the bus from release until acpiphp is unloaded.

Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Tested-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Reported-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>